It is well known that during development young animals and humans maintain the positive sodium balance that is required if both growth and maintenance of homeostasis are to occur concurrently. Multiple studies have shown that only a small fraction of the dietary sodium is excreted into the urine of growing subjects, even when the intake is so excessive that clinically apparent edema develops. Recently, it has been suggested that a high reabsorption of sodium in the distal tubule is, in part, responsible for the normal positive balance and the limited natriuresis observed following sodium expansion. The plasma renin activity and serum aldosterone concentration have also been shown to be high in normal neonates, however, the role of these substances in determining the positive balance and limited excretory capacity has not been tested. Moreover, the mechanisms by which the renin-angiotensin-aldosterone system is kept at a high level have not been studied. Finally, the influence of age related changes in the distribution of the body fluids on the tubular handling of sodium and the plasma renin activity have not been systematically scrutinized. We propose to examine the maturation of the negative feedback mechanisms, i.e., volume expansion and elevations in blood pressure during ontogeny. Correlations between these variables and the development of the renal handling of sodium will be sought.